Process for preparing low soda pseudoboehmite gel

ABSTRACT

A low soda pseudoboehmite gel is provided by reacting a NaAlO 2  solution with a NaHCO 3  solution having a concentration of less than 75 grams/liter and preferably less than 50 grams/liter. The soda content of the resulting AlO(OH) gel can be further lowered by repeated washing and/or redispersing (repulping) in water and/or further neutralization with a carboxylic acid having 1 to 6 carbon atoms.

BACKGROUND OF THE INVENTION

This invention relates to production of alumina gels and moreparticularly to a pseudoboehmite gel having a low sodium content.

The production of a low soda (Na₂ O) pseudoboehmite gel, i.e. having alow sodium content, would be desirable from the standpoint that lowerNa₂ O content results in a product with a higher Brunauer-Emmett-Tellersurface area and (H₂ O) adsorption capacity. Less Na₂ O in the gel alsomeans a savings in production cost from less Na₂ O being lost in theproduct and less acid, such as for example, acetic acid, being neededfor pH adjustment (Na₂ O removal). Furthermore, pseudoboehmite gelshaving high Na₂ O content are not easily formed into balls, waterbreakage increases and the crushing strength decreases. Furthermore, alow Na₂ O gel could be useful for certain catalytic applications inaddition to its use as a desiccant and adsorbent.

Pseudoboehmite gel is normally produced by reacting a sodium aluminate(NaAlO₂) solution with a hydrated acid salt of aluminum such asAlCl₃.6H₂ O, Al₂ (SO₄)₃.18H₂ O, or Al(NO₃)₃.9H₂ O or with the equivalentacids themselves. However, the use of acid processes, while producinglow soda gels, results in an acid salt by-product which is hard todispose of according to environmental regulations.

Edward, Frary and Jeffries in Aluminum and Its Production, McGraw-HillBook Company, Inc., New York (1930) at page 161 indicate that aluminacan be prepared from an aluminate solution using carbon dioxide gas orsodium bicarbonate (NaHCO₃). When sodium bicarbonate is used (i.e. analkaline process), it is conventional to use a concentrated solutionhaving approximately 80-100 grams/liter NaHCO₃ to provide sufficientneutralization of the basic sodium aluminate solution. The resultantproduct, however, can contain about 3-6% Na₂ O by total weight of theAl₂ O₃ (the calcined product of the pseudoboehmite gel). Quitesurprisingly, however, we have now discovered that a low Na₂ Opseudoboehmite gel can be produced having enhanced surface area as well,while still using an alkaline process.

SUMMARY OF THE INVENTION

In accordance with the invention, a pseudoboehmite gel having a Na₂ Ocontent of less than 3% by total weight of the calcined Al₂ O₃ productand characterized by a BET surface area of greater than 325 meter² /gramis produced by reacting a sodium aluminate solution with a sufficientamount of sodium bicarbonate solution having a concentration of lessthan 75 grams/liter, and preferably less than 50 grams/liter to provideapproximately 95% but less than 100% neutralization of the NaOH andNaAlO₂ in the solution.

BRIEF DESCRIPTION OF THE DRAWING

The sole drawing is a flowsheet illustrating the process of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the invention, a pseudoboehmite gel having a Na₂ Ocontent of less than 3% by total weight of the calcined Al₂ O₃ productis produced by reacting a sodium aluminate solution with a sodiumbicarbonate solution.

The term "pseudoboehmite" is used to describe the AlO(OH) precipitatedgel product which results from the neutralization of sodium aluminate.The term was derived to distinguish it from chemically similarcrystalline boehmite because the pseudoboehmite and the crystallineboehmite exhibit different x-ray diffraction patterns. Thepseudoboehmite exhibits a pattern of broad, diffused bands of α-AlO(OH)while the crystalline boehmite has a similar x-ray diffraction patternbut with distinct lines. The pseudoboehmite also usually exhibits a muchhigher Brunauer-Emmett-Teller (BET) surface area (approximately 300-600m² /g); whereas hydrothermally prepared boehmite has a much lowersurface area (usually less than 100 m² /g). Psuedoboehmite therefore maybe defined as an alumina having the chemical formula AlO(OH) which has aloss on ignition (LOI) at 1200° C. for 1 hour between about 20-30% byweight (boehmite=15.0% LOI) and contains at least about 50% by weight ofpseudoboehmite as determined by intensity of diffraction of copper(K_(a)) radiation (from the 020 planes), the corresponding interplanardistance being 6.5-6.8 Angstroms.

The sodium bicarbonate is used in a concentration of less than 75 g/land preferably at a concentration of approximately 10-50 g/l. The sodiumaluminate-NaOH solution used may be a synthetically produced solution(e.g., using aluminum trihydroxide) or may be the product of a Bayer orsinter process. A typical NaAlO₂ solution but not limiting to thisinvention, is about 100-400 g/l of total caustic at 0.6-0.8 Al₂ O₃/total caustic (as Na₂ CO₃) ratio. The more concentrated NaAlO₂solutions are more desirable since there is proportionately lessfiltrate to handle. When the concentration of Al₂ O₃ is about 80 gramsper liter of liquor at a 0.8 ratio, a solution of sodium bicarbonatehaving a concentration of 50 grams per liter can be used in a volumetricratio of approximately 3 liters sodium bicarbonate solution per liter ofliquor. However, the amount of sodium bicarbonate needed will vary inaccordance with the caustic content of the liquor. The total amount ofsodium bicarbonate used for the neutralization of the NaOH and NaAlO₂should not be sufficient to provide 100% neutralization. Preferably, theneutralization should not exceed about 95%; otherwise, dawsonite[NaAl(CO₃)(OH)₂ ] may form. By 95% is meant, 95% of the moles of Na+ inthe solution as NaOH and NaAlO₂. In fact, while we do not wish to bebound by any theory of operation, it is our belief that the surprisingresults achieved by this invention, i.e. use of lower concentrationNaHCO₃, than formerly done by the prior art, may result in lower amountsof Na₂ O in the gel due to inhibition of the formation of dawsonitewhich would otherwise be difficult to break down for further removal ofNa₂ O.

The sodium bicarbonate solution can be used at a temperature of 0° C. upto about 40° C., the decomposition temperature. The sodium aluminatesolution is maintained at a temperature from 70°-150° C. prior to mixingwith the sodium bicarbonate to avoid premature precipitation. A lowersolution temperature can be used if the NaAlO₂ contains a stabilizersuch as sodium gluconate. The solutions are blended together with mixeduntil approximately 95% of the Na+ is neutralized as indicated by a pHof about 11.2.

In accordance with the preferred embodiment of the invention, the gelslurry is filtered and residual Na₂ O removed with repeated washings ofthe filter cake with water (preferably deionized water), redispersed(repulped) one or more times in water, each time the slurry beingsubsequently filtered and washed, and then redispersed and the Na₂ Oneutralized with a carboxylic acid at a pH (of the repulp) of <10 toremove further amounts of Na₂ O and to further increase the surface areaof the resultant pseudoboehmite. The product is then filtered andwashed. Such organic acids include carbonic, formic, acetic, propionic,as well as any other organic acids, having 1-6 carbon atoms. Preferably,the amount of acid used during the final acid repulp should provide a pHrange of about 5-8 although improvements in the gel will be seen at a pHof less than 10. It should be noted that, although pH's <5 will workquite well for Na₂ O removal and increasing surface area, filtrationrates become slow.

It was quite surprising to find that at identical pH levels, repulpsusing organic acids resulted in higher surface area gels in contrast toinorganic acids (HCl, HNO₃, H₂ SO₄, H₃ PO₄ for example).

The amount of water used in the washes and repulps is typically about5-70 liters per kg of Al₂ O₃ but is not limited to this range.

The temperature of the repulp solution to which the acid is added may beany temperature about 0° C. and less than 100° C. However, the highertemperatures are preferred to expedite filtration. To further illustratethe invention, the following experiments were carried out.

EXAMPLE I

A sodium aluminate liquor was made up by atmospheric digestion of AlcoaC331 Bayer trihydrate [Al(OH)₃ ] with Mallinckrodt analytical grade NaOHpellets to provide a liquor having an analysis of 80 grams/liter Al₂ O₃and 100 grams/liter total caustic (computed as Na₂ CO₃) at a ratio of 80parts Al₂ O₃ per 100 parts total caustic.

The liquor was heated to 82° C. and then mixed with variousconcentrations of Baker reagent grade NaHCO₃ solution at 38° C. in astainless steel beaker. In each instance, a sufficient amount of NaHCO₃solution was used to provide 95% neutralization of the NaOH+NaAlO₂ inthe liquor. The resulting precipitation temperature in each instance wasabout 44° C., a typical pH of the resulting gel slurry was 11.2, and thefiltration temperature was 88° C. Each resultant gel filter cake waswashed with 5.4 ml of deionized water per gram of dry gel AlO(OH). Ineach instance the solids were repulped twice in deionized water at aratio of 51.7 grams of solid per liter of water, filtered and washed.The results are tabulated in Table I. Note that to obtain the desired95% neutralization, the total amount of NaHCO₃ was the same in eachinstance, only the concentration was varied.

                                      TABLE I                                     __________________________________________________________________________        Initial Gel       Precipi-                                                                           Precipi-                                               Slurry  g/l  ml   tation                                                                             tation                                                                              % LOI (1200° C.)                                                                   % Na.sub.2 O in                                                                        B.E.T. S.A.,            Sample                                                                            g/l Na.sub.2 CO.sub.3 *                                                               NaHCO.sub.3                                                                        NaHCO.sub.3                                                                        ° C.                                                                        pH    dried filter cake                                                                         Calcined Basis                                                                         m.sup.2 /g              __________________________________________________________________________    1   12      10   2000 38   10.92 21.50       1.04     420                     2   28      25   800  40   11.06 24.84       1.46     408                     3   41      40   500  42   11.19 26.28       1.81     342                     4   49      50   400  43   11.22 21.96       1.99     343                     5   65      75   267  48   11.28 22.33       2.56     337                     6   78      100  200  50   11.69 23.32       3.34     312                     __________________________________________________________________________     *Na.sub.2 CO.sub.3 formed after precipitation, excess NaAlO.sub.2 +NaOH       calculated as Na.sub.2 CO.sub.3.                                         

The results indicate a significant increase in the surface area andreduced Na₂ O content when 75 grams/liter or less NaHCO₃ was used.

EXAMPLE 2

To illustrate that lowering of the Na₂ O content in pseudoboehmite gelis not due solely to the lower Na₂ O content of the initial gel slurry,several runs were made with the sodium aluminate liquor of Example I and100 grams/liter NaHCO₃ in which various amounts of Na₂ SO₄ were added toraise the total Na₂ O concentration in the resulting initial gel slurry.The data clearly indicate that the solutions containing less NaHCO₃(samples 1-5) definitely result in a gel having a lower Na₂ O content aswell as increased surface area. In the case of samples 7 & 8, the totalNa₂ O content in the gel slurry was artificially raised (by the additionof sodium sulfate) yet the soda content and surface areas of the gelswere still approximately the same as sample 6, indicating that the lowersoda content is due primarily to useage of a lower concentration ofNaHCO₃, not simply a lowering of the total Na₂ O content of the initialgel slurry although the latter does have an influence.

                                      TABLE II                                    __________________________________________________________________________             Initial Gel   Precipi-                                                                           Precipi-                                              g/l  Slurry  g/l   tation                                                                             tation                                                                              % LOI (1200° C.)                                                                  % Na.sub.2 O in                                                                        B.E.T. S.A.,            Sample                                                                            NaHCO.sub.3                                                                        g/l Na.sub.2 CO.sub.3 *                                                               Na.sub.2 SO.sub.4 **                                                                ° C.                                                                        pH    dried filter cake                                                                        Calcined Basis                                                                         m.sup.2 /g              __________________________________________________________________________    5    75  65      0     48   11.28 22.33      2.56     337                     6   100  78      0     50   11.69 23.32      3.34     312                     7   100  93      33.3  53   11.37 22.99      3.51     291                     8   100  108     66.6  52   11.47 23.87      3.55     318                     __________________________________________________________________________     *Equivalent total Na.sub.2 O expressed as Na.sub.2 CO.sub.3 (i.e. sodium      aluminate plus sodium hydroxide plus sodium sulfate).                         **Added to NaHCO.sub.3 solution.                                         

EXAMPLE 3

To illustrate that extremely low Na₂ O gels can be made by a totallyalkaline process without the use of acid repulps or acid washes, ExampleI was repeated with 10 times more wash water. Table III shows that a0.02 % Na₂ O gel can be obtained when a low NaHCO₂ concentration is usedin conjunction with greater wash volumes.

                                      TABLE III                                   __________________________________________________________________________        Total 1 wash                                                                            % LOI (1200° C.)                                                                % Na.sub.2 O in Gel,                                                                  B.E.T. S.A.,                                   Sample                                                                            H.sub.2 O per kg Al.sub.2 O.sub.3                                                       dried filter cake                                                                      Calcined Basis                                                                        m.sup.2 g                                      __________________________________________________________________________    1   7.06      23.32    3.34    312                                            9   70.60     24.85    0.02    444                                            __________________________________________________________________________

The next example shows the beneficial results of organic acid repulps ingreatly increasing surface area and reduced Na₂ O content.

EXAMPLE 4

To further illustrate the invention with respect to the removal of Na₂ Oand increased surface area by redispersing or repulping with organicacids in a pH range of 5-10, a series of gels were made by firstreacting the NaAlO₂ liquor of Example 1 with a NaHCO₃ solution having aconcentration of 100 g/l. Each gel was then treated as in Example 1. Inthe final (second) repulp, the respective acid was added to the repulpwhich was at 88° C.

A number of inorganic acids were also used similarly on identical gelsas an additional comparison. The results in Table III show the superiorresults obtained when an organic acid, in a pH range of 5-10, is used ona gel formed by neutralization of a NaAlO₂ liquor with a NaHCO₂solution, in contrast to repulps made using inorganic acids or justrepulping with water.

                                      TABLE IV                                    __________________________________________________________________________             final                                                                         repulp                                                                            % LOI (1200° C.)                                                                % Na.sub.2 O in Gel,                                                                  B.E.T. S.A.,                                    Sample                                                                            Acid pH  dried filter cake                                                                      Calcined Basis                                                                        m.sup.2 /g                                      __________________________________________________________________________     6  none 10.40                                                                             23.32    3.34    312                                             10  acetic                                                                             10.00                                                                             23.52    2.77    380                                             11  acetic                                                                             9.00                                                                              25.18    2.58    395                                             12  acetic                                                                             8.00                                                                              24.00    2.29    423                                             13  acetic                                                                             7.00                                                                              23.59    1.90    501                                             14  acetic                                                                             6.00                                                                              29.99    1.39    567                                             15  acetic                                                                             5.00                                                                              29.74    0.98    553                                             16  propionic                                                                          5.00                                                                              28.43    1.22    569                                             17  formic                                                                             5.00                                                                              24.31    1.33    482                                             18  carbonic                                                                           7.00                                                                              21.95    2.08    358                                             19  HCl  5.00                                                                              24.05    1.94    421                                             20  HNO.sub.3                                                                          5.00                                                                              26.94    2.22    396                                             21  H.sub.2 SO.sub.4                                                                   5.00                                                                              26.97    2.36    392                                             22  H.sub.3 PO.sub.4                                                                   5.00                                                                              17.72    2.80    462                                             __________________________________________________________________________

Having thus described the invention, what is claimed is:
 1. A processfor making a pseudoboehmite gel having a BET surface area of at least325 m² /g and a Na₂ O content less than 3% by weight which consistsof:(a) reacting NaAlO₂ solution with a sufficient amount of a NaHCO₃solution having a concentration of less than 75 grams/liter toneutralize approximately 95% of the Na⁺ in said NaAlO₂ to form a low Na₂O content pseudoboehmite gel while inhibiting the formation ofdawsonite; (b) filtering the gel slurry to recover a gel filter cake;and (c) repulping and washing the resultant AlO(OH) product to furtherlower the Na₂ O content.
 2. The process of claim 1 wherein the AlO(OH)product is washed one or more times with water in an amount equal to5-70 liters per kilogram of Al₂ O₃ per treatment.
 3. The process ofclaim 2 wherein the washed AlO(OH) is repulped in water at a pH of 5-8and neutralized with a carboxylic acid containing 1-6 carbon atoms tofurther lower the Na₂ O content to less than 3% and/or to increase thesurface area.
 4. The process of claim 3 wherein the NaHCO₃ concentrationis less than 50 grams/liter.
 5. A process for making a pseudoboehmitegel having a BET surface area of at least 325 m² /g and a Na₂ O contentof less than 3% of the Al₂ O₃ weight which consists of:(a) reacting asolution containing 100-400 g/l caustic as Na₂ CO₃ and 0.6-0.8 grams Al₂O₃ per gram of caustic as Na₂ CO₃ with a sufficient amount of solutioncontaining less than 75 g/l NaHCO₃ to neutralize approximately 95% butless than 100% of the Na⁺ in the solution as NaOH and NaAlO₂ to form aAlO(OH) gel while inhibiting the formation of dawsonite; and (b) washingand/or redispersing (repulping) the AlO(OH) product with water in anamount equal to 5-70 l/kg of Al₂ O₃ one or more times.
 6. A process formaking a pseudoboehmite gel having a BET surface area of at least 325 m²/g and Na₂ O content less than 3% by weight which comprises:(a) reactinga NaAlO₂ solution with a sufficient amount of a NaHCO₃ solution having aconcentration of less than 75 grams/liter to neutralize approximately95% of the Na⁺ in said NaAlO₂ to form a low Na₂ O content pseudoboehmitegel while inhibiting the formation of dawsonite; (b) filtering the gelslurry to recover a gel filter cake; (c) pulping and washing theresultant AlO(OH) product to further lower the Na₂ O content; and (d)repulping the washed AlO(OH) in water at a pH of 5-8 and neutralizingwith a carboxylic acid containing 1-6 carbon atoms to further lower theNa₂ O content to less than 3% and/or to increase the surface area.